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I believe the model is a RGPS. Gas, updraft. I need to find the papers on it...

As for the registers.... there are 5 downstairs and 4 upstairs. The are all approximately
(did not measure).... 4" x 10" each. One downstairs is kept closed since it's only 4 feet from the T-Stat. The house is pretty small Sq Ft about 1100 or so.

I just remembered..... Here are some pics of the old furnace when we bought the house 17 years ago. I cleaned up a lot of that mess since. You're gonna love how the previous owner had the AC condensation drain set up!!

Without a return in the crawlspace, it would put your house into a negative pressure.

What would a negative pressure in the house do ?

I was thinking a negative pressure may have built up under the house that may have restricted the flow out the flu pipe. Therefore causing the pressure switch to activate and shutting down the system during startup. ?? Is that plausible ?

I'm not quite sure which one your referring to. ?? The flexible hose and the plenum (before the filter) are all new. The 2 - 12" ducts are from the old setup.

What would installing a supply run in the crawl space accomplish ?? More capacity perhaps ?

I was looking at the supply plenum on top of the a/c coil. In Pix 0306, On the back side it appears there is a 6" x ??? that runs about a foot and then goes into (what looks like) 8" x ??? short way elbow. I think you have much to much restriction there.

I was thinking a negative pressure may have built up under the house that may have restricted the flow out the flu pipe. Therefore causing the pressure switch to activate and shutting down the system during startup. ?? Is that plausible ?

How big is the crawlspace. And what BTU is the furnace again.

A negative pressure in the house will draw in the cold dry outside air. making you heating cost increase, and your house very dry and uncomfortable.